1. Field of Invention
The invention relates generally to the dicing of semiconductor wafers and, more particularly to the monitoring of blade location and flange clearance for safely cutting a semiconductor wafer.
2. Background Art
Die separation, or dicing, by sawing is the process of cutting a microelectronic substrate into its individual circuit die with a rotating circular abrasive saw blade. This process has proven to be the most efficient and economical method in use today. It provides versatility in selection of depth and width (kerf) of cut, as well as selection of surface finish, and can be used to saw either partially or completely through a wafer or substrate.
Wafer dicing technology has progressed rapidly, and dicing is now a mandatory procedure in most front-end semiconductor packaging operations. It is used extensively for separation of die on silicon integrated circuit wafers. Increasing use of microelectronic technology in microwave and hybrid circuits, memories, computers, defense and medical electronics has created an array of new and difficult problems for the industry. More expensive and exotic materials, such as sapphire, garnet, alumina, ceramic, glass, quartz, ferrite, and other hard, brittle substrates, are being used. They are often combined to produce multiple layers of dissimilar materials, thus adding further to the dicing problems. The high cost of these substrates, together with the value of the circuits fabricated on them, makes it difficult to accept anything less than high yield at the die-separation phase.
Dicing semiconductor wafers by sawing is an abrasive machining process similar to grinding and cutoff operations that have been in use for decades. However, the size of the dicing blades used for die separation makes the process unique. Typically, the blade thickness ranges from 0.6 mils to 500 mils, and diamond particles (the hardest well known material) are used as the abrasive material ingredient. Because of the diamond dicing blade's extreme fineness, compliance with a strict set of parameters is imperative, and even the slightest deviation from the norm could result in complete failure.
The diamond blade is a cutting tool in which each exposed diamond particle comprises a small cutting edge. Various dicing blades are available commercially. By way of example, a sintered diamond blade includes diamond particles which are fused into a soft metal such as brass or copper, or incorporated by means of a powdered metallurgical process; a plated diamond blade includes diamond particles which are held in a nickel bond produced by an electroplating process; and a resinoid diamond blade is one in which diamond particles are typically held in a resin bond to create a homogeneous matrix. Silicon wafer dicing typically uses the plated diamond blade, which has proven to be most successful for this application.
Because most state-of-the-art dicing equipment has been designed specifically to dice silicon wafers, problems arise when it is necessary to cut harder and/or more brittle materials. Blade speed and torque, depth of cut, feed rate, and other performance parameters have been optimized for silicon. However, hard and brittle materials require different blades and equipment operating parameters, the proper selection of which is a key to success for high-yield dicing. In any cutting operation, tool sharpness is of primary importance. More exactly, it is necessary that the cutting tool maintain its sharpness throughout the cutting operation. When cutting hard material such as sapphire or garnet, the cutting edges become dull quite rapidly. Because the dulled cutting edges cannot be re-sharpened in the usual manner, it is desirable that they be pulled loose from the blade, or else be fractured to expose new sharp cutting edges.
An important characteristic of the resinoid diamond blade that promotes effective cutting is its self-sharpening ability. The blade requires no dressing at all, in contrast to most metal-bonded (sintered or electroplated) diamond blades. Sharpening is accomplished automatically by the cutting process. As a cutting edge becomes dull, it experiences increased cutting forces that eventually either pull the diamond particle loose from the blade or else fracture it to produce a new sharp cutting edge. A diamond blade that does not exhibit this property cannot properly cut hard materials, nor can it perform properly if saw operating parameters interfere with the self-sharpening mechanism.
By way of example, U.S. Pat. No. 4,219,004 addresses a problem in the art of getting the blade cutting surface perpendicular to the substrate being cut and discloses blade mounting means comprising a pair of generally flat round collars, flanges, having a round central opening for receipt by the saw spindle. Further, the outer diameters of the collars are less than the blade diameter for providing an exposure of approximately 15 mils. A blade exposure not greater than 20 to 25 times the blade thickness is recommended. Replacing the collars with those having smaller diameter are disclosed for providing desired exposure and for replacing collars as the blade wears and exposure is reduced. Methods for monitoring or measuring the exposure during the dicing of the substrate is not addressed. U.S. Pat. No. 4,787,362 discloses an abrasive cutting blade having very high rigidity useful in dicing silicon wafers and hard materials. The use of the flange or spacer for maintaining blade rigidity and providing blade exposure sufficient for completely penetrating the work piece and cutting partially into the intermediate carrier typically used is disclosed. Wobble or run-out is of concern and is inversely proportional to the blade exposure. As a result, blade exposure is held to tight and typically minimal dimensions. A rigid blade core is described for preventing run-out from causing the core to make contact with the workpiece and causing widening of the cut and a less than even cut. Making the flange larger for providing less exposure is not addressed. However, less exposure means greater chance for inadequate cooling and greater chance of the flange hitting the work piece. There remains a need to effectively and economically resolve these problems. U.S. Pat. No. 3,987,670 discloses a displacement transducer manually applied to a diamond blade cutting surface for measuring a distance from the blade cutting edge to a fixed reference distance on the blade. The transducer is mounted on a portable fixture. Blade wear of diamond blades generally in the range of 18 to 36 inches are addressed and the problems associated with measuring blade wear of these blades are identified. The transducer is provided with suitable readout devices to determine blade wear. Although blade wear is addressed, it is for relatively large, easily visible blade sizes, and measured while the blade is held motionless. Further, the issues associated with exposure and depth of cut into a substrate is not addressed. Flange clearance is not a major concern for 18" to 36" blades.
There is a need to monitor blade exposure, the amount of blade extending from the flanges holding the blade therebetween, during a wafer or substrate dicing for maintaining sufficient clearance between the flange edges and the substrate to provide adequate cooling, and further for preventing the flanges from contacting the substrate, often containing electronic chips valued in the many thousands of dollars. There is further a need to monitor and control the location of the cutting blade with respect to the location of the wafer to be cut and to efficiently and effectively control positions prior to a first cut and during movement of the wafer on its table for subsequent cuts. By way of example, a dicing machine user will typically try to mount the wafer at the center of the table or chuck holding the wafer during the cutting operation. In the alternative, computer aided chuck and saw movement will determine measured cuts from the table center and move the dicing saw relative to the center coordinates, sometimes actually moving the table to the center prior to moving it to the appropriate cutting location. This adds expensive operating time, especially when one considers that thousands of cuts may be required within one wafer dicing project. When a cut is to be made close to an edge of the wafer, and the blade is allowed to make a cut close to the wafer edge, the blade may ripe off a section of the wafer, which can require disposal of the entire wafer, or extensive attempts and time for salvaging what is typically a very expensive wafer including multiple electronic elements.
Various approaches have been used to identify a locations of a workpiece in computer aided machines. By way of example, U.S. Pat. No. 4,233,625 to Altman discloses the use of television monitoring for aligning successive configurations of semiconductors. U.S. Pat. No. 5,422,579 to Yamaguchi discloses the use of a camera for identifying probe positions on a card and recognizing reference probes for providing a corrective movement to a work table. U.S. Pat. No. 4,819,167 to Cheng et al. discloses a system and method for determining the location of a semiconductor wafer relative to its destination position using an array of optical sensors positioned along an axis transverse the path of movement of the wafer. Trigger points provided by the sensor array as the wafer is moved, provide locus information data to a processor for calculating the center of the wafer. U.S. Pat. No. 3,670,153 to Rempert et al. discloses the use of a light sensing element and scanning of the object for detecting dark and light regions in determining edges of the object to be measured. In spite of the many computerized optical devices and configurations, there still remains a need to economically provide a method for effectively and efficiently locating the position of the wafer on the work table for optimizing movement of the table or workpiece during sawing operations and for providing a safe location at which the saw can operate without damage to the wafer and saw, or hazard to the saw and operator.